<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <p>局部变量计数。</p>
    <button type="button" onclick="myFunction()">计数!</button>
    <p id="demo">0</p>
    <script>
        function myFunction() {
            var demo = document.getElementById("demo").innerHTML
            demo++
            document.getElementById("demo").innerHTML = demo
        }
    </script>
</body>

</html>